At present, data communications using Wireless Local Area Networks (WLANs) have become very common, and the demand for WLAN coverage across the world is increasing. The Institute of Electrical and Electronic Engineers (IEEE) industry standard 802.11 group has defined a series of standards such as 802.11a/b/g/n/ac to meet the increasing communication needs.
In an Orthogonal Frequency Division Multiplexing (OFDM) system, the basic structure of a radio frame is shown in FIG. 1. The radio frame includes two parts: a physical layer preamble and a data section. The physical layer preamble includes a preamble compatible with traditional equipment, a short training domain, a long training domain, a signaling field and so on. To reduce the overhead, the IEEE 802.11 protocol defines a Null Data Packet (NDP), which refers to a physical layer packet that contains only the physical layer preamble but does not contain the data section.
The basic architecture of a WLAN may refer to a Basic Service Set (BSS), including an Access Point (AP) and multiple stations (STAs) associated with the AP, as shown in FIG. 2.
If working areas of access points in two BSSs overlap and the operating frequency bands used by the access points/stations are the same or overlap, the two BSSs are called as Overlapping Basic Service Set (OBSS).
When multiple wireless stations share channels, conflict detection in the wireless environment becomes very difficult. One of the major problems is hidden stations. Referring to FIG. 3, a station A sends data to a station B, and at the same time a station C also sends data to the station B. Because both station C and station A are outside the coverage of each other, simultaneous sending of data from station A and station C will lead to conflict. From the station A's perspective, the station C is a hidden station. In order to solve the problem of hidden stations, IEEE 802.11 proposed a virtual channel detection mechanism which avoids the collision of hidden stations by including a time for reserving a channel in the frame header of a radio frame. Other listening stations that receive the radio frame containing the time information for reserving the channel set a locally stored Network Allocation Vector (NAV), the value of NAV is set as the maximum of the time for reserving the channel and reserved time information. During this time, the listening stations will not send data, so as to avoid channel competition with the hidden stations and thereby collision. After NAV is reduced to zero, other stations can send data. In addition, before sending of big data, the sending party can send a Request to send (RTS) to make a channel reservation, which includes the time for reserving the channel. The receiving party feeds back a Clear to Send (CTS) to acknowledge the channel reservation, which also includes the time for reserving the channel, so as to protect the radio frame(s) subsequently sent by the sending party. As shown in FIG. 4, the time length between the time point when the sending of the RTS is completed and the time point when sending of the CTS begins, the time length between the time point when the sending of the CTS is completed and the time point when the sending of data begins, and the time length between the sending of the data is completed and the time point when the sending of ACK (acknowledgement) begins are all short interframe space (SIFS). For other stations, the NAV of the RTS is the time length between the time point when the sending of the RTS is completed and the time point when the sending of the ACK is completed, and the NAV of the CTS is the time length between the time point when the sending of the CTS is completed and the time point when the sending of the ACK is completed.
In addition, as WLAN deployment becomes more and more intensive, collision between BSSs is getting worse and worse, and OBSS problem is more and more serious. Referring to FIG. 5, a station A and an access point A belong to a basic service set A, a station B and an access point B belong to a basic service set B, and the basic service set A and the basic service set B are OBSS. If the station A competes for the channel to send data to the access point A, since the station A and the station B are hidden stations for each other, the station B may continue to compete for the channel to send data at this time. If the station B sends data during the sending of data from the station A, data collision will occur. The access point B cannot correctly receive the radio frame from the station B at this time, and the current transmission and receiving of the station A can be influenced.
This section provides background information related to the present disclosure which is not necessarily prior art.